feat(admin): module skeleton, config and two database connections

Adds the admin module: env config with fail-fast validation, two MongoDB
connections (its own vantage_admin database plus a narrow path into the
control plane), the boot sequence and the image.

Config refuses to start without a signing key, and both Mongo URIs must
name their database inline -- admin talks to two databases, so a bare
MONGO_DB would be ambiguous about which.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

admin/.dockerignore

@@ -0,0 +1,2 @@
+.env
+*.lic